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A Novel Technique to Determine the Adsorption Capacity and Breakthrough Time of Adsorbents Using a Noncontact High Resolution Microwave Resonator Sensor Mohammadreza Fayaz, Mohammad Hossein Zarifi, Mohammad Abdolrazzaghi, Pooya Shariaty, Zaher Hashisho, and Mojgan Daneshmand Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03418 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Environmental Science & Technology

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A Novel Technique to Determine the Adsorption Capacity

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and Breakthrough Time of Adsorbents Using a Non-contact

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High Resolution Microwave Resonator Sensor

4

Mohammadreza Fayaz1, Mohammad H. Zarifi2, Mohammad Abdolrazzaghi2, Pooya

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Shariaty1, Zaher Hashisho1* and Mojgan Daneshmand2

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1

Department of Civil and Environmental Engineering, University of Alberta, Canada, T6G 1H9

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2

Department of Electrical and Computer Engineering, University of Alberta, Canada, T6G 1H9

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*Corresponding author: Tel.:+1-780-492-0247; Fax:+1-780-492-0249; E-mail: [email protected]

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Abstract

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A newly developed non-contact high resolution real-time microwave sensor was used to

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determine the breakthrough time and adsorption capacity for adsorbents/adsorbates with different

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dielectric properties. The sensor is a microwave microstrip planar resonator with enhanced

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quality factor using a regenerative feedback loop operating at 1.4 GHz and adjustable quality

14

factor of 200 to 200,000. Beaded activated carbon (BAC, microwave absorbing) and a polymeric

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adsorbent (V503, microwave transparent) were completely loaded with 1,2,4-trimethylbenzene

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(non-polar) or 2-butoxyethanol (polar). During adsorption, variations in the dielectric properties

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of the adsorbents were monitored using two microwave parameters; quality factor and resonant

18

frequency. Those parameters were related to adsorption breakthrough time and capacity.

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Adsorption tests were completed at select relative pressures (0.03, 0.1, 0.2, 0.4, and 0.6) of

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adsorbates in the influent stream. For all experiments, the difference between the breakthrough 1 ACS Paragon Plus Environment


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time (t5%) and the settling time of the quality factor variation (time that the quality factor was

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0.95 of its final value), was less than 5%. Additionally, a linear relationship between the final

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value of the resonant frequency shift and adsorption capacity was observed. The proposed non-

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contact sensor can be used to determine breakthrough time and adsorption capacity.

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TOC/Abstract graphic

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Introduction

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Adsorption is a useful technique for controlling emissions of volatile organic compounds

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(VOCs) since it allows their recovery and reuse.1-3 Following adsorption, the loaded adsorbent

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should be regenerated to restore its adsorption capacity and recover the adsorbed VOCs.4

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Breakthrough time, which is the time required for the outlet concentration to reach 1%-5% of the

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inlet concentration, has been recognized as a criterion for ending the adsorption process and

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switching to the regeneration.5 Direct measurement of the effluent concentration during

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adsorption is typically used to determine breakthrough time.6-8 To perform the breakthrough-time

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measurement, the monitoring instruments should be directly in contact with the effluent stream,

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which might be hazardous for the user or might contaminate and damage the detectors in the

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presence of high boiling point and toxic or corrosive compounds.7, 9 Therefore, introducing a

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technique for non-contact determination of breakthrough time could be industrially relevant.

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The performance of an adsorption system depends on the adsorption capacity of the adsorbent.

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Adsorption capacity for different concentrations of a VOC can be expressed in the form of an

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adsorption isotherm.10 Adsorption isotherms for some VOCs on different adsorbents (e.g.,

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activated carbon or zeolite) could be obtained using several techniques such as dynamic column

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method,11 static volumetric method,12 gravimetric method,13 and sometimes a combination of

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these methods.14 All the aforementioned techniques used in the studies on determination of

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adsorption isotherms used adsorbates with low to moderate boiling points such as acetaldehyde,

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methyl ethyl ketone, benzene, toluene, xylene and cumene.2, 13-18 Obtaining adsorption isotherms

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for high boiling point adsorbates or corrosive/toxic compounds can be challenging as these

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compounds can contaminate/damage instruments and/or pose health risk.



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Microwaves signals are electromagnetic waves in the frequency range of 0.3-300 GHz.

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Microwave techniques have demonstrated a significant potential in areas related to

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environmental engineering, such as microwave heating for adsorbent regeneration and

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microwave sensing.6, 19-23 Microwave sensing uses a non-contact method to monitor the variation

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of the dielectric properties of materials in the vicinity of the sensor, which makes the sensor

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attractive for use in harsh environments (e.g., corrosive or toxic gases). Recent studies

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demonstrated that the residual lifetime of an activated carbon, used in adsorption of water vapor,

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can be estimated by monitoring the variations in its dielectric properties.19, 24

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Mason et al. developed a microwave cavity resonator to determine the residual lifetime of

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activated carbon exposed to water vapor. For this purpose the variation in the permittivity of

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carbon was correlated to its exposure time to water vapor. However, the cavity-based resonator

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sensor used in that study had moderate quality factor and was in direct contact with the carbon

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and adsorbate, which can restrict its applications in some harsh and noisy environments.24 Rebel

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et al. developed an in-situ sensing device based on impedance measurement to measure water

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vapor adsorption capacity of granular activated carbon by monitoring capacitance variation in an

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electronics circuit.19 Staudt et al. used a combination of gravimetric and impedance analysis to

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measure dielectric properties of gas molecules in adsorbed phase, and correlate them to the

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adsorbed amount.25 The reported sensors in the previous studies have simple structure and are

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low cost; however, they are more vulnerable to noise26 than the resonant-based counterparts

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since they are operating based on capacitive sensing method. Recently microwave planar

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resonators have been widely used due to their various range of applications, complementary

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metal oxide semiconductor (CMOS) compatibility for on-board chip process, easy design and

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fabrication, and low cost. Study of planar microwave resonators utilizing split ring resonators has 4 ACS Paragon Plus Environment

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been widely popular in sensing applications due to their moderately high quality factors and

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small size.27, 28 Their high performance in non-contact sensing also made planar resonators more

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useful for gas sensing applications.29-32 Microwave planar sensors can be coated with an

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adsorbent to estimate the concentration of VOCs. de Fonsecaa et al. used planar sensor coated

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with different zeolites as sensitive material to detect different concentrations of toluene in

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measurement atmosphere.33 These resonators are sensitive to variation in dielectric properties of

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the surrounding medium. Specifically, during adsorption, any change in the loading state of the

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adsorbents can be translated into a change in their electrical properties that result in shift in

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resonant frequency as well as the quality factor of the resonator’s frequency response.24, 34, 35 The

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significantly high electromagnetic loss in planar microwave resonators leads to a low to

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moderate quality factor and consequently low resolution in sensing applications. In order to

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resolve such an issue, a newly developed active sensor is used where the loss of system is

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compensated using an active feedback.35, 36 This constructive regeneration of power cancels out

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the effect of high loss and improves the quality factor for orders of magnitude.

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The objective of this study is to investigate the application of a microwave resonator sensor for

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measuring breakthrough time and adsorption capacity of adsorbents loaded with high boiling

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point VOCs. This sensor utilizes a very high quality factor microwave resonator for contactless

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sensing of the permittivity change in the adsorbent.31 During adsorption, the variations in the

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sensor’s resonant frequency and quality factor are measured and monitored using a vector

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network analyzer. These changes are used to determine the breakthrough time and adsorption

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capacity of a carbonaceous (microwave absorbing or lossy) and a polymeric (microwave

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transparent) adsorbents loaded with VOCs. Two high boiling point VOCs with different

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polarities are used as adsorbates. Adsorbents and adsorbates with contrasted dielectric properties 5 ACS Paragon Plus Environment


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are selected to demonstrate the effectiveness of the proposed technique and device for

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determining the breakthrough time in non-contact way.

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Experimental

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Adsorbents and Adsorbates

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Beaded activated carbon (BAC, Kureha Corporation) and beaded polymeric adsorbent (V503,

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DOWEX Optipore, DOW Chemical Company) are two adsorbents used in this work. BAC is a

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microwave absorbing (lossy) adsorbent, and V503 is a microwave transparent (low loss)

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adsorbent. Both adsorbents are in the form of spherical beads, and widely used for capturing

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VOCs from industrial streams.7, 37 For BAC, BET surface area and total pore volume are 1339

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m2/g and 0.56 cm3/g, respectively.35 For V503, BET surface area and total pore volume are 963

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m2/g and 0.79 cm3/g, respectively.35 1,2,4-trimethylbenzene (98%, Sigma-Aldrich) and 2-

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butoxyethanol (99%, Acros Organics) were used as adsorbates. These two compounds are

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common VOCs emitted during painting and surface coating operations.38, 39 Both compounds

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have the same boiling point (171 °C); however, BE is polar while TMB is non-polar. Therefore,

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the dielectric constant and loss factor of BE are higher than of TMB.40 The contrasted dielectric

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properties of the select adsorbents and adsorbates allow testing the performance of the sensor for

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an adsorbent/adsorbate system with low permittivity (i.e., V503/TMB) and a system with high

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permittivity (i.e., BAC/BE).

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Experimental Setup and Methods

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Adsorption 6 ACS Paragon Plus Environment

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The experimental setup is presented in Figure 1a. The setup consisted of an adsorption tube,

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adsorbate vapor generation system, gas detection system and data acquisition and control system.

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The tube was made of quartz with 2.2 cm inner diameter and 35.6 cm length. Prior to each

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adsorption experiment, it was filled with 4.1±0.1g of the adsorbent. A fritted glass disk held the

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adsorbent bed in place. The vapor generation system consisted of a syringe pump (KD Scientific,

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KDS-220) that injected liquid adsorbate into a dry air stream to achieve the target concentration.

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The air flow rate was initially set to 10 standard liters per minute (SLPM, 25 °C and 1 atm) using

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a mass flow controller (Alicat Scientific). The gas detection system consisted of a

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photoionization detector (PID, Minirae 2000, Rae Systems) that monitored concentration of the

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adsorbate at the tube’s outlet. The measured concentration was linearly correlated to a voltage

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signal, sent to DAC, and recorded by LabVIEW program. The PID was calibrated before each

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experiment using the adsorbate stream, generated with the vapor generation system. When the

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outlet concentration reached the inlet concentration, the adsorbent was considered to be

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saturated. For each adsorbate, the inlet concentrations in the inlet stream were selected according

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to the adsorbate relative pressure, of 0.03, 0.1, 0.2, 0.4 and 0.6. Using the ideal gas low, the

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liquid injection rate of the syringe pump was calculated so that the inlet concentration of the

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adsorbate in the air stream was equal to the corresponding relative pressure. The data acquisition

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system consisted of a LabVIEW program (National Instruments) and a data logger (National

135

Instruments, Compact DAC) equipped with analog input and output modules to record outlet

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VOC concentration.

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After saturation, adsorption capacity was determined using Equation (1): Adsorption capacity g/g =

(1)



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where WAA is the adsorbent weight after adsorption and WBA is the adsorbent weight before

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adsorption.

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All gravimetric measurements were conducted while the reactor was capped to avoid any

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adsorbate loss.

BAC Sample Active Feedback

Resonator

(b)

(a) 142

Figure 1 (a) Schematic diagram of the adsorption setup and, (b) Picture of the microwave

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resonator sensor used for non-contact testing of BAC

144 145

Dielectric properties measurements

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During adsorption, the dielectric properties of virgin adsorbents and adsorbates affect the slope

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and direction of the resonant frequency (i.e., up or down frequency shift). Therefore, the

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dielectric properties of adsorbents and adsorbates should be measured to qualitatively verify the

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experimental results from the proposed sensor.

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The apparatus used to acquire complex permittivity (dielectric properties) of materials under test

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(MUT) consists of a Vector Network Analyzer (VNA, supplied by Rohde and Schwarz),

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equipped with an open-ended coaxial probe (Keycom), and data acquisition software.

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Measurements of the solvents were performed at 25 °C and 1 atmosphere. After placing the


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sample in a quartz tube (2 cm inner diameter, 2 cm length), dielectric property measurements

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were conducted from 500MHz to 2GHz. Reflection coefficients of the electromagnetic waves

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from MUT at each frequency were measured. The material affects the phase and magnitude of

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the reflected power observed by the VNA, from which the complex permittivity is calculated.

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Sensor analysis

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A microwave planar open-ended resonator sensor is used to monitor the changes in the dielectric

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properties of BAC and V503 during adsorption of VOCs. The resonant profile of the sensor (S21-

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parameter) is observed and recorded during adsorption process and the main parameters such as

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resonant frequency (fr) and quality factor (Q) are extracted from the resonant profile. The

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resonant frequency can be defined as the frequency where the maximum power transmission

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occurs. The quality factor is a ratio of the resonant frequency to the -3 dB bandwidth of the

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resonant profile as Q = fr/∆f3dB. In the developed sensor, the resonant frequency is initially

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designed at 1.42 GHz with a controllable quality factor. The quality factor can be controlled and

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adjusted in a range of 200 to 200000.32

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The sensor used in this study is a half wavelength resonator, illustrated in Figure 1b. The

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resonant frequency is related to the effective permittivity of the wave-propagation environment

170

according to Equation 2.35 =

(2)

171

where c = 3×108 (m/s) is the speed of light, fr (Hz) the resonant frequency, εeff_the effective

172

permittivity of the wave-propagation environment, and λg (m) represents the guided wavelength.



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During adsorption the changes in the dielectric properties of adsorbent are detected by the sensor

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through measuring the changes in quality factor and resonant frequency. The environmental

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conditions around the sensor were kept stable during all experiments (e.g., no temperature

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change nor mechanical shock around the setup). For each test, steady state results before starting

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and after finishing adsorption confirmed the stable environmental conditions around the sensor.

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The passive resonator coupled with transmission line was turned into an active resonator by

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introducing a positive feedback loop. The loop acts as a negative resistor to the device and

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returns the lost energy due to radiation or lossy substrate back into the system, resulting in a high

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quality factor resonator.41 The utilized microwave sensor was fabricated on a RO5880 (Roger

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Corporation) substrate with permittivity of 2.2, loss tangent of 0.0009 and thickness of 0.787

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mm.42 Also, the thickness of the copper trace on the sensor was 35 microns.

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The effective distance that the sensor is sensitive to the changes in the dielectric properties is 8

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cm. During adsorption, the sensor was placed at a fixed distance of 1 cm away from the tube and

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the sensing area was 1 cm from the bottom of the adsorbent bed (Figure 1b). The position of the

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sensor with respect to the tube was selected so that the measurements were conducted with the

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highest possible accuracy. The resonator along with the adsorbent could then be described using

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a simple circuit model, expressed as a parallel capacitor and a resistor, where the changes in the

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capacitance and resistance could be described by variations in the resonant frequency and quality

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factor provided by the sensor, respectively. During adsorption, the dielectric properties of an

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adsorbent/adsorbate system, exposed to microwaves, change according to Cole and Cole

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equation.25 The equivalent complex permittivity in the medium is described as ε = ε" − jε""

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where ε" is the dielectric constant which is related to the equivalent capacitor, and ε"" is the

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dielectric loss factor which is related to the equivalent resistor. Thus, the changes in the quality 10 ACS Paragon Plus Environment

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factor and resonant frequency of the sensor could be obtained from the changes in the dielectric

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properties of the loaded adsorbent. These changes are represented by the changes in the

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scattering parameters measured using VNA.35 According to Equation 2, as the effective

199

permittivity of the medium increases, the resonant frequency shifts downward. Such an ultrahigh

200

quality factor reduces the minimum detectable permittivity variation of materials according to

201

Equation 3 and increases the resolution of the device in permittivity sensing. This enables the

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non-contact high precision operation of the sensor35:

*√,

|∆ε'() | = -

./01 2

× √4kTBR

(3)

203

where Q is the quality factor (-), k is the Boltzmann constant (1.38×10-23 m2kg s-2K-1), T is the

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room temperature (K), B is the measured bandwidth (Hz), R is the resistance (Ω), 9 is the

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complex permittivity (-), and Vomax is the maximum amplitude of the resonance profile (V).

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All the tests were completed in duplicates, and the average results with standard deviations are

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presented. However, the standard deviation values were so small that they might not be clearly

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observed in the figures (i.e., ≤6 kHz and ≤0.011 g adsorbate/g adsorbent for frequency and

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adsorption capacity, respectively).

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Results and Discussion

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A comparison between the simulated resonant profiles in passive and active states of the

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resonator sensor is demonstrated in Figure 2a. The simulation is performed in high frequency

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software simulator (HFSS) based on finite element method and the electric filed distribution

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around the sensor is shown in Figure 2b. Electric field distribution around the sensor is an

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important factor in determining the performance of the sensor in non-contact operations. 11 ACS Paragon Plus Environment


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Changes in the dielectric properties of materials in the near vicinity of the sensor can alter the

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electric field and changes in the electric field can be translated to changes in the frequency and

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quality factor in the response profile of the sensor. Electric field simulation illustrates strong hot-

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spots on the microstrip resonator at the resonant frequency. These hot spots are the most suitable

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positions for placing the tested materials and allow the sensor to achieve the maximum

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sensitivity. E field (V/m)

(a) 222 223

(b)

Figure 2 (a) Comparison between active and passive response profile in Full Wave Simulation, (b) Electric Field distribution in surface of sensor at 1.42 GHz

224 225

Breakthrough time

226

Figure 3a shows the location of the adsorbent bed with respect to the sensor. During adsorption,

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the accumulation of the adsorbed molecules in liquid-like phase in the adsorbent as well as gas

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concentration between the adsorbent beads change the dielectric properties of the loaded

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adsorbent. The changes in the dielectric properties of the adsorbent are detected in terms of

230

quality factor and resonant frequency shift (i.e., ∆Q and ∆fr, respectively). The downward

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direction of gas flow during adsorption resulted in saturation of the adsorbent bed from top to

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bottom. The extension of the saturation zone through the bed (from point a to b in Figure 3a) 12 ACS Paragon Plus Environment

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alters the electric field of the sensor and creates more shift in the quality factor and resonant

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frequency. Figure 3b shows variations in the relative quality factor and outlet concentration

235

during adsorption of 2-butoxyethanol on V503 for different relative pressures. For different

236

relative pressures, when the saturation zone approaches the end of the bed, the quality factor

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stops changing (i.e., ∆Q = ∆Qfinal). Meanwhile, the adsorbent gets saturated and the outlet

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concentration starts increasing. Therefore, the time that the quality factor stops changing is

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similar to the breakthrough time.

) ) Relative Concentration(C/C (C/Cinlet initial )) Q Factor (∆Q/∆Q (∆Q/∆Qfinal P/P0Relative =0.6 final 1.00

Inlet Gas

0.75 0.50

Adsorption Bed

0.25

P/P0=0.03

0.00 1.00 0.75 0.50 0.25

a

Sensor

P/P0=0.1

0.00 1.00

b

0.75 0.50

Outlet Gas

0.25

P/P0=0.2

0.00 1.00 0.75

PID

0.50 0.25

P/P0=0.4

0.00 1.00 0.75 0.50 0.25

P/P0=0.6

0.00

0

100

200

300

400

500

Time (min)

(b)

(a) 240

Figure 3 Changes to the effluent concentration (expressed as C/Cinlet) and quality factor

241

(expressed as ∆Q/∆Qfinal) during adsorption of 2-butoxyethanol on V503 13 ACS Paragon Plus Environment

600


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tQ is defined as the time that ∆Q reaches 95% of ∆Qfinal, and breakthrough time (tb) is defined as

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the time that the outlet concentration reaches 5% of the inlet concentration (i.e. when C/Cinlet

244

=0.05). Figure 4 indicates that for all experiments in this study, regardless of the adsorbent and

245

adsorbate type, the difference between tb and tQ is less than 5%. Therefore, breakthrough time

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can be estimated by monitoring the changes in the quality factor.

(a)

(b)

300

400 y = 1.04x - 1.67 R² = 0.99

y = 1.04x - 1.89 R² = 0.99

300 tb (min)

tb (min)

200

200

100 100 0

0 0

100 200 tQ (min)

300

0


100

200 300 tQ (min)

400

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(d)

(c) 500

300 y = 1.01x - 0.59 R² = 0.99

300

y = 0.97x + 0.55 R² = 0.99

200

tb (min)

tb (min)

400

200

100 100 0

0 0

100

200 300 tQ (min)

400

500

0

100

200

300

tQ (min)

247

Figure 4 tb versus tQ for adsorption of (a) 1,2,4-trimethylbenzene on BAC (b) 1,2,4-

248

trimethylbenzene on V503 (c) 2-butoxyethanol on BAC and (d) 2-butoxyethanol on V503

249 250

Adsorption capacity

251

The high quality factor of the resonator enables it to detect very small changes in the dielectric

252

properties of the sensitive area in front of the sensor based on the shift of the resonant frequency.

253

Therefore, in addition to adsorbed VOC, the VOC concentration in the space between the beads

254

affects permittivity in the environment and consequently, the resonant frequency. Adsorption,

255

however, results in more significant shift in resonant frequency; because after adsorption VOC

256

state changed from gas phase to liquid-like phase in the adsorbent pores.35 When the saturation

257

zone reaches the sensitive area, the resonant frequency gradually increases until the adsorbent

258

reaches complete saturation (Figure 5).



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Relative Concentration (C/C (C/Cinletinitial ) ) )) Relative Resonance Frequency Shift (∆f (∆f/∆f r/∆fr,final final

1.00 0.75 0.50 0.25

P/P0=0.03

0.00 1.00 0.75 0.50 0.25

P/P0=0.1

0.00 1.00 0.75 0.50 0.25

P/P0=0.2

0.00 1.00 0.75 0.50 0.25

P/P0=0.4

0.00 1.00 0.75 0.50 0.25

P/P0=0.6

0.00 0

200

400

600

800

Time (min)

Figure 5 Change to the concentration and relative frequency shift during adsorption of 2butoxyethanol on V503 259

Figure 6 illustrates the equilibrium adsorption capacities and resonant frequency shifts at

260

different relative pressures of 1,2,4-trimethylbenzene and 2-butoxyethanol adsorbing on BAC

261

and V503. The higher adsorption capacities for V503 at higher relative pressures could be

262

attributed to 44% higher total pore volume of V503 compared to that for BAC.



Adsorption capacity (q, g/g)

600

0.4

400

0.2

200

0.0

0 0.0

0.1

0.2

0.3

0.4

0.5

(b)

0.6

0.4

400

0.2

200

0.0

0

0.6

0.0

0.1


263

600

0.4

400

0.2

200

0.0

0 0.1

0.2

0.3

0.4

0.5

Resonant frequency shift (∆f, kHz) Adsorption capacity (q, g/g)

(c)

0.0

0.2

0.3

0.4

0.5

0.6

Partial pressure (P/P0)

Partial pressure (P/P0) 0.6

600

Resonant frequency shift (∆f, kHz)

(a)

0.6

Resonant frequency shift Resonant frequency shift (∆f, kHz) Adsorption capacity (q, g/g)

Adsorption capacity

0.6

(d)

0.6

600

0.4

400

0.2

200

0.0

0 0.0

0.1


0.2

0.3

0.4

0.5


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0.6


264

Figure 6 Adsorption capacity and resonant frequency shift versus partial pressure for (a) 1,2,4-

265

trimethylbenzene on BAC (b) 1,2,4-trimethylbenzene on V503 (c) 2-butoxyethanol on BAC and

266

(d) 2-butoxyethanol on V503

267

During the adsorption process, the variation in the permittivity of the adsorbent depends on the

268

number of adsorbate molecules (i.e., dipoles) in the adsorbent bed because the interactions

269

between microwaves and the adsorbate molecules in the system are responsible for polarizing the

270

molecules, consequently changing the dielectric properties of the adsorbent and shifting the

271

resonant frequency of the sensor.25, 43 The changes in dielectric properties, however, are more

272

considerable for polar molecules with permanent dipole moments. Figure 7 shows that 2-

273

butoxyethanol with a polar molecular structure has higher permittivity than 1,2,417 ACS Paragon Plus Environment


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trimethylbenzene with a non-polar molecular structure. Nevertheless, non-polar compounds are

275

also affected by microwaves through inducing dipole moments in the molecules; therefore,

276

during adsorption of non-polar adsorbate onto the adsorbent, the resonant frequency also

277

changes.25

278

For BAC/1,2,4-trimethylbenzene system, the resonant frequency shift does not significantly

279

change with relative pressure due to the lower permittivity of 1,2,4-trimethylbenzene compared

280

to that of BAC (Figure 7). The low dielectric properties of 1,2,4-trimethylbenzene could be

281

attributed to its non-polar molecular structure while the high dielectric properties of BAC is due

282

to the presence of delocalized π electrons, resulting in enhanced polarization interactions with

283

microwaves.44 Since 2-butoxyethanol and BAC have comparable effective permittivities, at low

284

inlet concentrations resonant frequency shift only changes due to adsorbed 2-butoxyethanol (i.e.

285

low gas concentration did not have significant contribution to the resonant frequency change).

286

For higher concentrations of 2-butoxyethanol, however, the resonant frequency changed more

287

noticeably because more 2-butoxyethanol is present in the adsorbed phase as well as in the gas

288

phase.

289

The effective permittivity of V503 is lower than that of 1,2,4-trimethylbenzene and 2-

290

butoxyethanol; therefore, when V503 was loaded with either adsorbates, the resonant frequency

291

notably changed. Because 2-butoxyethanol has higher effective permittivity than 1,2,4-

292

trimethylbenzene, the change in the resonant frequency for V503 loaded with 2-butoxyethanol is

293

more notable than for V503 loaded with 1,2,4 trimethylbenzene.



BAC V503

4

BAC V503

1.0

3 2

0.5

1 0.0

0 0.5

1.0

1.5

2.0

0.5

1.0

1.5

10

Dielectric Constant (ε')

1.5

BE TMB

BE TMB

8

2.0

3

2

6 4

1

2

Loss Factor (ε")

Dielectric Constant (ε')

5

Loss Factor (ε")

Page 19 of 26

0

0 0.5

1.0

1.5

2.0

0.5

Frequency (GHz)

1.0

1.5

2.0

Frequency (GHz)

294 295

Figure 7 Dielectric properties of BAC, V503, 2-butoxyethanol (BE), and 1,2,4 trimethylbenzene

296

(TMB)for the frequencies between 0.5 to 2.0 GHz

297 298

For each adsorbent/adsorbate system, increasing the adsorbed mass increased the resonant

299

frequency shift. Therefore, it is expected that for different relative pressures, there is a

300

correlation between the final resonant frequency shift and adsorption capacity. Figure 8 shows

301

that for each system, the final resonant frequency shift linearly increased with adsorption

302

capacity. BAC has higher permittivity compared to 1,2,4-trimethylbenzene; therefore, a change

303

in adsorption capacity has resulted in a small change in the sensor’s resonant frequency shift. For

304

the other adsorbent/adsorbate scenarios, the adsorbate has comparable or higher dielectric

305

properties than the corresponding adsorbent; consequently, the changes in the resonant frequency

306

are more noticeable. As a result, by developing such calibration curves, changes in adsorption

307

capacity could be monitored more distinctly for low permittivity adsorbents, loaded with

308

adsorbates with comparable or higher permittivities. It should be noted that since ∆fr is zero for

309

the blank adsorbents, the general correlations between q and ∆fr should include the origin as a 19 ACS Paragon Plus Environment


Page 20 of 26

310

data point. A linear relationship holds within the tested relative pressure range; however, a non-

311

linear relationship should be used for a wider pressure range.

312

V503

400

(b)

(a)

200

300

150

R2= 0.88 y=380x-6

R2= 0.98 y=612x-31

100 R2= 0.97 y=104x-30

50

200

R2= 0.96 y=7992x-3206

100

0 0.3



BAC

0

0.4

0.5

0.6

0.3


0.4

0.5

0.6


Figure 8 Resonant frequency shift versus adsorption capacity of (a) 1,2,4-trimethylbenzene and (b) 2-butoxyethanol on BAC and V503 313

314

The results obtained in this study are encouraging as they show the potential of the sensor for

315

monitoring breakthrough time and adsorption capacity during adsorption. The sensor could be

316

used in a full-scale adsorber by positioning it close to the end of the adsorbent bed. Alternatively

317

several sensors could be placed along the length of the bed to monitor the progression of

318

adsorption. However, additional research is still needed to better understand and characterize the

319

performance of the sensor. For instance, additional research is needed to verify the performance

320

of the sensor under different operating parameters such as temperature, flowrate, composition.

321

Increasing the adsorption temperature is expected to reduce the adsorption capacity and

322

consequently decrease the permittivity change in the adsorbent bed, which results in small


Page 21 of 26


323

changes in the sensor’s characteristics (quality factor and resonant frequency). While single

324

VOCs were used in this study, it is expected that the sensor will detect the overall effect of the

325

mixtures, based on the dielectric properties of the components, in a similar way that a flame

326

ionization detector (FID) or a PID detects the overall/effective concentration depending on how

327

the different components generate ions during combustion in a hydrogen flame (for FID) or UV

328

photoionization (for PID). While an FID or PID cannot differentiate the different gas species in a

329

mixture, they are still very useful and commonly used analytical devices. Additional research is

330

also needed to understand the performance of the sensor in presence of non-target adsorbates

331

(e.g., the presence of water vapor in a VOC mixture).

332

The reported sensor and the sensing method can potentially be used in conjunction with portable

333

and mobile devices. As such, VNA can be replaced by a low cost voltage controlled oscillator

334

and a frequency to voltage converter can potentially be used as the readout circuitry. This device

335

can be integrated with smart-phones for wireless data gathering purposes, which demonstrates

336

the reliability, low-cost, and real-time operation of the device with long lifetime.

337

Acknowledgement

338

The authors acknowledge financial support from the Natural Science and Engineering Research

339

Council (NSERC) of Canada. We also acknowledge infrastructure and instruments grants from

340

the Canada Foundation for Innovation (CFI), NSERC, and Alberta Advanced Education and

341

Technology.



342

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A Novel Technique for Determining the Adsorption ... - ACS Publications

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